A comparison of different cluster mass estimates: consistency or discrepancy?

Rich and massive clusters of galaxies at intermediate redshift are capable of magnifying and distorting the images of background galaxies. A comparison of different mass estimators among these clusters can provide useful information about the distribution and composition of cluster matter and its dynamical evolution. Using the hitherto largest sample of lensing clusters drawn from the literature, we compare the gravitating masses of clusters derived from the strong/weak gravitational lensing phenomena, from the X-ray measurements based on the assumption of hydrostatic equilibrium, and from the conventional isothermal sphere model for the dark matter profile characterized by the velocity dispersion and core radius of galaxy distributions in clusters. While there is excellent agreement between the weak lensing, X-ray and isothermal sphere model-determined cluster masses, these methods are likely to underestimate the gravitating masses enclosed within the central cores of clusters by a factor of 2–4 as compared with the strong lensing results. Such a mass discrepancy has probably arisen from the inappropriate applications of the weak lensing technique and the hydrostatic equilibrium hypothesis to the central regions of clusters, as well as from assuming an unreasonably large core radius for both luminous and dark matter profiles. Nevertheless, it is pointed out that these cluster mass estimators may be safely applied on scales greater than the core sizes. Namely, the overall clusters of galaxies at intermediate redshift can still be regarded as the dynamically relaxed systems, in which the velocity dispersion of galaxies and the temperature of X-ray emitting gas are good indicators of the underlying gravitational potentials of clusters.     

Lunar occultation of Saturn: III. How big is Iapetus?

By considering both the orbital lightcurve of Iapetus and data obtained during the March 30, 1974, occultation of the satellite by the Moon, we obtain information about the brightness distribution on the bright face of Iapetus and derive an accurate value for the satellite's radius. From the observed orbital lightcurve we find that the trailing face of Iapetus must consist predominantly of a single bright material with an effective limb-darkening parameter of k = 0.62_?0.12^0.10. Given this result the occultation observations imply a radius of 718_?78⁺⁸⁷ km. If the patchy albedo model proposed by Morrison et al. represents the surface of Iapetus accurately (as far as the relative albedo distribution is concerned) then the radius of Iapetus is 724 ± 60 km. Both estimates are consistent with the radiometric radius of 835 (+50, ?75) km derived by Morrison et al. Combining our results with the value of 0.60 ± 0.14 for the normal reflectance (in V) of the material at the center of the bright face derived by Elliot et al. we find that the normal reflectance of the dark side material is 0.11_?0.03^+0.04. These values are higher than the corresponding values of 0.35 and 0.05 quoted by Morrison et al.     

Strike-slip fault patterns on Europa: Obliquity or polar wander?

Variations in diurnal tidal stress due to Europa’s eccentric orbit have been considered as the driver of strike-slip motion along pre-existing faults, but obliquity and physical libration have not been taken into account. The first objective of this work is to examine the effects of obliquity on the predicted global pattern of fault slip directions based on a tidal-tectonic formation model. Our second objective is to test the hypothesis that incorporating obliquity can reconcile theory and observations without requiring polar wander, which was previously invoked to explain the mismatch found between the slip directions of 192 faults on Europa and the global pattern predicted using the eccentricity-only model. We compute predictions for individual, observed faults at their current latitude, longitude, and azimuth with four different tidal models: eccentricity only, eccentricity plus obliquity, eccentricity plus physical libration, and a combination of all three effects. We then determine whether longitude migration, presumably due to non-synchronous rotation, is indicated in observed faults by repeating the comparisons with and without obliquity, this time also allowing longitude translation. We find that a tidal model including an obliquity of 1.2°, along with longitude migration, can predict the slip directions of all observed features in the survey. However, all but four faults can be fit with only 1° of obliquity so the value we find may represent the maximum departure from a lower time-averaged obliquity value. Adding physical libration to the obliquity model improves the accuracy of predictions at the current locations of the faults, but fails to predict the slip directions of six faults and requires additional degrees of freedom. The obliquity model with longitude migration is therefore our preferred model. Although the polar wander interpretation cannot be ruled out from these results alone, the obliquity model accounts for all observations with a value consistent with theoretical expectations and cycloid modeling.     

High-Resolution 0.33-0.92 μm Spectra of Iapetus, Hyperion, Phoebe, Rhea, Dione, and D-Type Asteroids: How Are They Related?

New high-resolution spectra in the 0.33 to 0.92 μm range of Iapetus, Hyperion, Phoebe, Dione, Rhea, and three D-type asteroids were obtained on the Palomar 200-inch telescope and the double spectrograph. The spectra of Hyperion and the low-albedo hemisphere of Iapetus can both be closely matched by a simple model that is the linear admixture of the spectrum of a medium-sized, high-albedo icy saturnian satellite and D-type material. Our results support an exogenous origin to the dark material on Iapetus; furthermore, this material may share a common origin and a similar means of transport with material on the surface of Hyperion. The recently discovered retrograde satellites of Saturn (Gladman et al., Nature412, 163-166) may be the source of this material. The leading sides of Callisto and the Uranian satellites may be subjected to a similar alteration mechanism as that of Iapetus: accretion of low-albedo dust originating from outer retrograde satellites. Phoebe does not appear to be related to either Iapetus or Hyperion. Separate spectra of the two hemispheres of Phoebe show no identifiable global compositional differences.     

Autonomous software: Myth or magic?

We discuss work by the eSTAR project which demonstrates a fully closed loop autonomous system for the follow up of possible micro‐lensing anomalies. Not only are the initial micro‐lensing detections followed up in real time, but ongoing events are prioritised and continually monitored, with the returned data being analysed automatically. If the “smart software” running the observing campaign detects a planet‐like anomaly, further follow‐up will be scheduled autonomously and other telescopes and telescope networks alerted to the possible planetary detection.We further discuss the implications of this, and how such projects can be used to build more general autonomous observing and control systems. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Westerlund 1: bound or unbound?

The Galactic open cluster Westerlund 1 (Wd 1) represents the ideal local template for extragalactic young massive star clusters, because it is currently the only nearby young cluster with a mass of ～10⁵?M_⊙. Its proximity makes spatially resolved studies of its stellar population feasible, and additionally permits direct comparison of its properties with measurements of velocity dispersion and dynamical mass for spatially unresolved extragalactic clusters. Recently, we published the dynamical mass estimate based on spectra of four red supergiants. We have now identified six additional stars which allow a determination of radial velocity from the wavelength covered in our VLT/ISAAC near-infrared spectra (CO bandhead region near 2.29 μm), significantly improving statistics. Using a combination of stepping and scanning the slit across the cluster center, we covered an area which included the following suitable spectral types: four red supergiants, five yellow hypergiants, and one B-type emission-line star. Our measured velocity dispersion is 9.2 km?s^?1. Together with the cluster size of 0.86 pc, derived from archival near-infrared SOFI-NTT images, this yields a dynamical mass of 1.5×10⁵ M_⊙. Comparing this to the mass derived via photometry, there is no indication that the cluster is currently undergoing dissolution.     

BV centauri: dwarf or classical nova?

The similarity in properties of GK Per and BV Cen suggests that the latter may be a classical nova remnant rather than a dwarf nova. Our H and V CCD photometry show no sign of a nova shell but disclose the presence of a close and probably physical companion. BV(RI) _c CCD photometry of the companion, assumed to be a dwarf, givesM _v =3.1±0.3 for BV Cen at quiescence. Even though half of the light in BV Cen comes from the secondary this is much brighter than expected for a dwarf nova and supports the conclusion that BV Cen was an unrecorded nova.     

The extreme, red afterglow of GRB 060923A: distance or dust?

Gamma-ray bursts (GRBs) are powerful probes of the early Universe, but locating and identifying very distant GRBs remain challenging. We report here the discovery of the K -band afterglow of Swift GRB 060923A, imaged within the first hour post-burst, and the faintest so far found. It was not detected in any bluer bands to deep limits, making it a candidate very high- z burst  ( z ≳ 11)  . However, our later-time optical imaging and spectroscopy reveal a faint galaxy coincident with the GRB position which, if it is the host, implies a more moderate redshift (most likely   z ≲ 2.8  ) and therefore that dust is the likely cause of the very red-afterglow colour. This being the case, it is one of the few instances so far found of a GRB afterglow with high-dust extinction.     

H_2O Megamasers: Accretion Disks, Jet Interaction, Outflows or Massive Star Formation?

The 25 years following the serendipitous discovery of megamasers have seen tremendous progress in the study of luminous extragalactic H₂O emission. Single-dish monitoring and high-resolution interferometry have been used to identify sites of massive star formation, to study the interaction of nuclear jets with dense molecular gas and to investigate the circumnuclear environment of active galactic nuclei (AGN). Accretion disks with radii of 0.1–3 pc were mapped and masses of nuclear engines of order 10⁶–10⁸ M were determined. So far, 50 extragalactic H₂O maser sources have been detected, but few have been studied in detail.     

Nebular or parent body alteration of chondritic material: Neither or both?

Abstract— Discussions of meteorite properties often concern whether they are the result of “nebular” or “parent body” processes, but several decades of research have not resolved the issue. Part of the problem is that any gas-phase reaction involving meteoritic materials thought to have occurred is automatically assumed to be nebular, even though the most primitive solar system objects are water- and volatile-rich and could easily generate vapors. Reactions important in meteorite genesis are those involving (1) oxidation of Fe, (2) oxidation of other cations, (3) reduction, (4) olivine-pyroxene equilibria, and (5) hydration of silicates. The P-T-x conditions required are almost invariably incompatible with standard models for the early solar nebula, but clearly many of these reactions occurred prior to final agglomeration. Most of the reactions require high pressures (>10^?3 atm) or, more importantly, major departures from cosmic composition, even though the final rocks are remarkably cosmic in elemental proportions. We suggest that most of these reactions occurred in a regolith rendered “dynamic” by the flow of copious volatiles. In such a scenario, liquid and gas phase reactions can occur at elevated temperatures and pressures relative to the nebula and with noncosmic gas phase compositions; but the system is “closed” to most components, so that cosmic proportions are essentially preserved.     

Glasses in howardites: Impact melts or pyroclasts?

We have analyzed glasses in eight howardites with the aim of distinguishing their origins as impact melts or pyroclasts. Although theoretical calculations predict that pyroclastic eruptions could have taken place on Vesta, the occurrence of pyroclastic glasses in HED meteorites has not been documented. This study involved petrographic examination of textures, electron microprobe analysis of major and minor elements, and LA‐ICP‐MS analysis for selected trace elements. Previously documented textural and compositional differences between lunar impact‐melt and pyroclastic glasses partly guided this study. This work yielded no positive identification of pyroclastic glasses. The most likely explanation is that pyroclastic glasses never formed, either because Vesta contains insufficient volatiles to have powered explosive eruptions, or because eruptive conditions produced optically dense fire‐fountains that allowed melt drops to collect as lava ponds. The impact‐melt glasses were grouped (low‐alkali, Ca‐rich, and K‐rich) based on compositions. We suggest that these glasses are the result of impacts onto known HED lithologies. The low‐alkali glasses are impact melts of bulk HED lithologies. We hypothesize that the Ca‐rich and K‐rich glasses result from oversampling of plagioclase and of mesostasis that experienced liquid immiscibility, respectively, during micrometeorite impacts into eucrite targets.     

Axions: Bose Einstein condensate or classical field?

The axion is a motivated dark matter candidate, so it would be interesting to find features in Large Scale Structures specific to axion dark matter. Such features were proposed for a Bose Einstein condensate of axions, leading to confusion in the literature (to which I contributed) about whether axions condense due to their gravitational interactions. This note argues that the Bose Einstein condensation of axions is a red herring: the axion dark matter produced by the misalignment mechanism is already a classical field, which has the distinctive features attributed to the axion condensate (BE condensates are described as classical fields). This note also estimates that the rate at which axion particles condense to the field, or the field evaporates to particles, is negligible.     

The active galaxy NGC 4151: Archetype or exception?

Summary. The Seyfert galaxy NGC 4151 harbors in its nucleus the most intensively studied AGN (Active Galactic Nucleus). Among the brightest AGN (in apparent luminosity) it is the most widely variable and the variations of its ultraviolet and X-ray spectrum have been studied on time scales ranging from hours to decades. These observations have formed the basis of methods and models which have been found to generally apply to broad emission line AGN: the rich and complex relation between the X-ray and UV variations, the comptonization model of the X-ray spectrum from medium X-ray to -rays, the reverberation mapping, the stratification in velocity and physical conditions of the gas in the broad line region, and a method to estimate the black hole mass from emission line variability. The large barred spiral which hosts this nucleus has been extensively studied especially in the central region. Inflow of gas along the and possibly also the orbits have been detected, but since the accretion disk is not in the galactic plane (as evidenced by the significant angle separating the radio axis and the rotation axis of the galaxy) the incoming gas seen on kpcs scale must, as it flows further inward, move out of the galactic plane, along trajectories which are entirely unknown. There is more to learn on NGC 4151. In fact, the best is yet to come. Three avenues of investigation appear particularly promising: 1) The variations in flux and spectral shape of the X-ray continuum and its relationship with the UV variations are the key to understanding the specifics of the Comptonization model. Progress on this point will come from repeated simultaneous observations of the UV spectrum and of the entire X-ray and -ray spectrum. This will also give insights on the structure of the disk in the last stable orbits, the formation and structure of the corona and in the end, the process of energy production. Exciting results on these topics are expected in the near future from Chandra-AXAF, XMM and INTEGRAL. The Chandra and XMM (which have short energy range) main contributions will, however, be line diagnostics and for Chandra, imaging of the soft diffuse emission. 2) The search for the gas inflow which merges into and/or forms the torus could finally be successful. Several powerful approaches are possible: observing molecular lines in emission with millimeter arrays of increasing baseline and collecting area; using the nuclear radio structure as background source to observe free-free and atomic or molecular lines in absorption. 3) The observations of NGC 4151 during a state of deep minimum will provide a unique oportunity to observe the X-ray spectrum of a Seyfert 1 nucleus at epochs of very low accretion rate, to identify the nature of the narrow variable lines, to determine the stellar population of a currently active nucleus, and measure the mass of the black hole from the stellar lines. NGC 4151 at minimum states should be a target of opportunity for all space missions. In addition, observations on time scales of 10 years or more, especially following a deep minimum, will allow one to map emitting regions of size up to 1pc, thereby overlapping with the linear scale directly mapped with large radio telescopes. Received 30 October 1999 / Published online: 24 March 2000     

Solar Intranetwork Magnetic Elements: Intrinsically Weak or Strong?

The high spatial resolution observation of a quiet region taken with the Solar Optical Telescope/Spectro-Polarimeter aboard the Hinode spacecraft is analyzed. Based on the Milne?CEddington atmospheric model, the vector magnetic field of the quiet region is derived by fitting the full Stokes profiles of the Fe i 630 nm line pair. We extract intranetwork (IN) region from the quiet region and identify 5058 IN magnetic elements, and study their magnetic properties. As a comparison, the magnetic properties of network (NT) region are also analyzed. The main results are as follows. i)?The IN area displays a predominance of weak (hecto-gauss) field concentration, i.e., 99.8?% of IN area shows the weak field. Moreover, the vector magnetic field exhibits concentration toward horizontal direction. However, for the NT region, we discover the coexistence of weak field and strong (kilo-gauss, kG) field. In the IN and NT regions, the filling factor shows almost the same probability distribution function with the peak at about 0.28. ii)?All IN magnetic elements show field strength less than 1?kG. However, some NT elements display the coexistence of weak field and strong field. Regardless of NT or IN elements, about 20?% of elements lies in the Doppler blue-shift region. Our results imply that not all magnetic elements lie in the down-draft sites.     

Gamma-ray pulsars: polar cap or outer gap emission?

The power law spectra of the six known-ray pulsars are seen to cut off at energies < 1 TeV so that no steady pulsed TeV component is observed from any known-ray pulsar. In some cases we show that the cutoff is steeper than exponential, which is consistent with magnetic pair production above the polar cap region. The small upper limits on the ratio of TeV energy flux to optical - GeV energy flux is again consistent with the polar cap model, but appears to be inconsistent with inverse Compton controlled outer gaps.     

Asteroid “clans”: Super-families or multiple events?

Among the recently determined asteroid families (Zappala' et al., 1992) a number of very large clusterings are recognized. Some of them agglomerate few families previously identified as single groupings. In most cases, the new big families (called clans) seem to be composed by different sub-clusterings connected each other by very narrow bridges. In the present paper we analyze their possible origin and evolution: a primordial single event followed by subsequent collisions of the fragments, a super-catastrophic original impact or different collisional events overlapping by chance.     

Superbubbles in magnetized interstellar media: blowout or confinement?

The importance of the interstellar magnetic field is studied in relation to the evolution of superbubbles with a three-dimensional (3D) numerical magnetohydrodynamical (MHD) simulation. A superbubble is a large supernova remnant driven by sequential supernova explosions in an OB association. Its evolution is affected by the density stratification in the galactic disc. After the superbubble size reaches 2–3 times the density scaleheight, it expands preferentially in the z -direction, until finally it can punch out a hole in the gas disc (blowout). On the other hand, the magnetic field running parallel to the galactic disc has the effect of preventing it from expanding in the direction perpendicular to the field. The density stratification and the magnetic fields have completely opposite effects on the evolution of the superbubble. We present results of a 3D MHD simulation in which both effects are included. As a result, it is concluded that when the magnetic field has a much larger scaleheight than the density, even for a model in which the bubble would blow out from the disc if the magnetic field were absent, a magnetic field with a strength of 5 μG can confine the bubble in | z |≲300 pc for ≃ 20 Myr (confinement). In a model in which the field strength decreases in the halo as B  ∝ ρ^1/2, the superbubble eventually blows out like a model with B  = 0 even if the magnetic field in the mid-plane is as strong as B  = 5 μG.     

Devolatilization or melting of carbonates at Meteor Crater,AZ?

We have investigated the carbonates in the impact melts and in a monolithic clast of highly shocked Coconino sandstone of Meteor Crater, AZ to evaluate whether melting or devolatilization is the dominant response of carbonates during high‐speed meteorite impact. Both melt‐ and clast‐carbonates are calcites that have identical crystal habits and that contain anomalously high SiO₂ and Al₂O₃. Also, both calcite occurrences lack any meteoritic contamination, such as Fe or Ni, which is otherwise abundantly observed in all other impact melts and their crystallization products at Meteor Crater. The carbon and oxygen isotope systematics for both calcite deposits suggest a low temperature environment (<100 °C) for their precipitation from an aqueous solution, consistent with caliche. We furthermore subjected bulk melt beads to thermogravimetric analysis and monitored the evolving volatiles with a quadrupole mass spectrometer. CO₂ yields were <5 wt%, with typical values in the 2 wt% range; also total CO₂ loss is positively correlated with H₂O loss, an indication that most of these volatiles derive from the secondary calcite. Also, transparent glasses, considered the most pristine impact melts, yield 100 wt% element totals by EMPA, suggesting complete loss of CO₂. The target dolomite decomposed into MgO, CaO, and CO₂; the CO₂ escaped and the CaO and MgO combined with SiO₂ from coexisting quartz and FeO from the impactor to produce the dominant impact melt at Meteor Crater. Although confined to Meteor Crater, these findings are in stark contrast to Osinski et al. (2008) who proposed that melting of carbonates, rather than devolatilization, is the dominant process during hypervelocity impact into carbonate‐bearing targets, including Meteor Crater.